Mask insert



FIG. 1 is a top perspective view of a first embodiment of a mask insert showing my new design;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a left side elevational view thereof;

FIG. 6 is right side elevational view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof.

The broken lines shown in the drawings illustrate portions of the mask insert and form no part of the claimed design. 

CLAIM The ornamental design for a mask insert, as shown and described. 